erlang, 148 bytes

Result

AC

Duration

1931ms

Code [DL]

%%!
main(_)->f(0).
f(X)->C=io:get_chars("",1),Y=case C of"T"->0;"K"->X-50;eof->halt(0);_->X+1 end rem 51,io:put_chars(if Y<1->"@";true->C end),f(Y).

stdin

               T                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
      K                                           

stdout

               @                                  
               @                                  
               @                                  
               @                                  
               @                                  
               @                                  
               @                                  
               @                                  
               @                                  
               @                                  
               @                                  
               @                                  
               @                                  
               @                                  
               @                                  
               @                                  
               @                                  
               @                                  
               @                                  
               @                                  
               @                                  
               @                                  
               @                                  
               @                                  
               @                                  
               @                                  
               @                                  
               @                                  
               @                                  
               @                                  
               @                                  
               @                                  
      @@@@@@@@@@                                  

stderr

strace